1. Field of the Invention
The present invention relates to a manufacturing method adapted for use in manufacturing a molded gear having chamfered portions which are formed with slopes provided on one end edge of each of a plurality of teeth. More particularly, the present invention relates to a manufacturing method of a molded gear which makes it possible to produce a highly accurate molding die by separately forming a gear piece corresponding to tooth width portions of the molded gear and a chamfer-application piece corresponding to chamfered portions; and which intends to accurately manufacture the molded gear in which the chamfered portions are provided on an end of each of the tooth, by using the molding die.
2. Description of the Related Art
Heretofore, a manufacturing method of a toothed gear having chamfered portions formed in each tooth end, has been generally performed by cutting with a cutting tool such as an end mill, which is attached to rotate while chamfering the edge of each tooth end of a gear member that is fixedly mounted on a work table of a machine tool (for example, refer to Japanese Unexamined Patent Publication No. 10-94921). Such manufacturing method with machining is effective in the case where the gear member is made of metal. However, in a case where the gear member is resin such as plastic, it is difficult to obtain good machining accuracy because the gear member is soft and it is likely to be high cost because of a number of machining steps included.
Consequently, in the case where the gear member is a soft member such as resin, the gear provided with chamfered portions on a tooth end is manufactured by performing injection-molding of resin, using a molding die previously provided with slope portions in response to one end of the gear teeth. For example, a manufacturing method of a molded gear, in which a plurality of teeth 1 as shown in FIG. 7 are provided and chamfered portions 2 are formed with slopes provided on one end edges of each of the teeth 1, forms first an electric discharging electrode member 3 having tooth profile portions 4 which has the same shape as cross-sectional shape perpendicular to a rotation axis of the molded gear and which has the same shape as a tooth profile of the tooth 1 shown in FIG. 7 on the circumferential surface thereof, as sown in FIG. 8A (a first step). Next, as shown in FIG. 8B, a discharging electrode 6 having chamfered slope portions 5 of the same shape as the chamfered portions 2 of the tooth 1 of the molded gear is formed by cutting tip ends of the respective end edges of the tooth profile portion 4 of the discharging electrode member 3 that was produced at the first step, by the use of a cutting tool such as an end mil (a second step). Further, by the use of the discharging electrode 6 formed at the second step, a molding die 8 as shown in FIG. 9, is formed in a metal member 7 having a thickness larger than the dimension of the tooth width of the gear to be molded by conducting the electrical discharge-machining (i.e., an electrical discharge-machining of a bottomed recess), so that machining is proceeded into the metal member 7 by a predetermined depth in a direction of an arrow A shown in the same FIG. 9 (a third step). At this time, a machined recess portion 10 having grooves 9 along the inner circumferential surface thereof coincident with the tooth profile portions 4 of the discharging electrode 6 is formed in the metal member 7, the recess portion 10 being configured to define a complementary relationship with the discharging electrode 6. In addition, slope portions 11 corresponding to the chamfered slope portions 5 formed on the end edges of the tooth profile portion 4 of the discharging electrode 6 are formed in respective ends of the grooves 9 which are located on the bottom side of the recess portion 10. Next, the molded gear provided with a plurality of the teeth 1, each having the chamfered portion 2 formed at an end portion thereof as shown in FIG. 7 is manufactured by performing injection-molding of resin using the molding die 8 (a fourth step).
However, in such a manufacturing method of the molded gear, as shown in FIG. 9, since the molding die 8 is formed by conducting the electrical discharge-machining for forming bottomed recess in the metal member 7 using the discharging electrode 6, which has the chamfered slope portions 5 of the tooth profile portion 4 at the tip end thereof, it becomes difficult to remove machined chips and powders generated during the electrical discharge remaining at the tip end of the discharging electrode 6 as processing depth becomes deeper, so that processing speed at the tip end must be lowered. Meanwhile, at a rear end of the discharging electrode 6, since the machined chips and powders by the electrical discharge machining is smoothly removed, electrical discharge-machining can proceed; well as a result, a change in the amount of processing by the electrical discharge machining between the tip end and rear end of the discharging electrode 6 occurs and machining accuracy of the molding die 8 is likely to be degraded. Therefore, even when a molded gear having the chamfered portions 2 using such the molding die 8 is manufactured, it is impossible to obtain a product with good accuracy.